Messages in this thread | | | Subject | Re: general protection fault in find_device | From | Anand Jain <> | Date | Tue, 26 Jun 2018 17:17:48 +0800 |
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(Sorry for the delay in replay due to my vacation).
Thanks Nikolay. more below.
On 06/18/2018 09:43 PM, Nikolay Borisov wrote: > > > On 18.06.2018 16:32, David Sterba wrote: >> On Mon, Jun 18, 2018 at 10:03:18AM +0300, Nikolay Borisov wrote: >>> So this suggests some inconsistency on fs_devices->devices list. On a >>> quick look indeed it doesn't seem clear what the locking rules for this >>> list are. In device_list_add in the !device case a device is added with >>> fs_devices->device_list_Mutex held and using list_add_rcu. In the same >>> function if we want to read the list ie invoke find_devices (because we >>> have found an fsid) we are using plain list_for_each_entry (ie not the >>> _rcu version and i don't see device_list_mutex being held while >>> iterating the list). Additionally in btrfs_free_extra_devids the >>> fs_devices->devices list is iterated with uuid_mutex being held and not >>> device_list_mutex. In open_fs_devices we don't get any protection >>> whatsoever while reading the list. >> >> The uuid_mutex or device_list_mutex is provided by a caller up the >> stack. >> >>> Same thing in >>> btrfs_find_next_active_device. If the list is supposed to be >>> RCU-protected then the rules are: >>> >>> 1. There needs to be an out of band (ie not RCU) mutual exclusion of >>> modifiers >> >> that's device_list_mutex for fs_devices::devices >> >>> 2. Iterating the list should use _rcu list primitives. >>> >>> Currently I don't see those 2 invariants being enforced in every code path. >> >> Where is it not enforced for example? > > Admittedly I didn't check the whole call chain but for example in > find_device it's used "naked". Perhaps putting some lockdep_assert in > various places dealing with fs_devices->devices list would help ? >> >> If the device_list_mutex is held, list traversal does not use >> list_for_each_entry_rcu, otherwise it does (eg the DEV_INFO ioctl or >> btrfs_show_devname). >> >> The problem that triggers this report is IMO in device_list_add that >> uses the device list unprotected. Anand sent patches for that, but they >> were titled as 'cleanups' so I skipped them for the merge window.
Ah. sorry to confuse you. Will consolidate fixes into github (also reviewing David's fixes as well) and will use syz to confirm.
Thanks, Anand
>> Candidate fixes are: >> >> https://patchwork.kernel.org/patch/10437705/ >> https://patchwork.kernel.org/patch/10437713/ > Yep those 2 definitely look like fixing unlocked accesses to > fs_devices->devices list
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